Generally, 3-dimensional display refers to the technology of adding depth information to a 2-dimensional image and using this depth information to allow the viewer to feel a sense of 3-dimensional vividness and reality. Various types have been proposed of typical 3-dimensional display devices in prior art according to such technologies, in a variety of forms and methods. Until now, most of these technologies display 3-dimensional images using the principle of binocular disparity of a human being. As there are slight deviations between images presented to the left eye and to the right eye, perception of the disparity due to the left and right eyes creates a sense of 3-dimensionality, so that a sense of protrusion may be obtained.
A typical form of such technique in prior art is to separate the left and right images, mainly with or without using eyeglasses. Using glasses are the anaglyph type, polarized glasses type, and liquid crystal shutter type, while without glasses are the lenticular sheet type, parallax barrier type, and optical plate type. Among these conventional technologies, the polarized glasses type is the oldest and most stable 3D display type, and is most widely used in 3-dimensional movies and 3-dimensional monitors, etc. The biggest drawback of this method, however, lies in the requirement of using special polarized glasses for 3-dimensional images. The lenticular sheet type and parallax barrier type, among the types not using eyeglasses in prior art, provide low brightness and low resolution images and entail a fixed viewing position for a viewer, to cause headaches or dizziness when viewing for an extended period of time. There are also complete 3-dimensional types, including the holographic and volumetric 3D display types. While these types can produce 3-dimensional images freely in a space, they require expensive laser and precision optical components to display even a still image, and cannot provide real-time 3-dimensional images.
On the other hand, as methods for solving these problems, some non-glasses types have been proposed that utilize reflectors, conventional optical lenses, and concave mirrors, etc., to enable real-time 3-dimensional images at lower costs. However, most of these methods experience distortion of images due to the concave mirrors, etc., and high costs of manufacturing when large devices are used. In particular, when large devices are used in order to obtain a large display, there is a need to form a very large width of space, which is a large obstacle with regard to the utility and applicability of these types.
In addition to these methods using concave mirrors and reflectors, methods using Fresnel lenses, such as in the present invention, have been proposed in various types for a long time. As in U.S. Pat. No. 3,537,771 (granted Nov. 3, 1970), it has been disclosed that two Fresnel lenses can be used to result in a 3-dimensional image effect. In particular, as in U.S. Pat. No. 5,782,547 (granted Jul. 21, 1998), it has been disclosed that one or two or more Fresnel lenses and reflectors, etc., can be used to create 3-dimensional images in a form having background images. These technologies have the drawbacks of increased manufacturing costs due to the use of several independent image sources and half mirrors for obtaining several background images, and of the large volume of space required by the overall structure of the device, which are large obstacles to their commercialization.
Referring to FIG. 1, which illustrates a 3-dimensional display device based on prior art, an image projected from an object image source supply part 5 is reflected by means of a reflector 6, and is reflected again by a half mirror 4 to be projected towards a second Fresnel lens 1. The second Fresnel lens 1 and a first Fresnel lens 2 work in combination like a single lens, so that a 3-dimensional image 9 appears within the focal length region of the double Fresnel lenses (the second Fresnel lens 1 and the first Fresnel lens 2). Also, in order to obtain a background image, an image projected from a background image source supply part 7 is transmitted through the half mirror 4 which removes light from the second Fresnel lens 1 and from the outside, and is transmitted through a dark filter 8 which increases the 3-dimensional image effect, to be positioned behind the 3-dimensional image 9 as a background image to the 3-dimensional image 9. This is to allow the expression of a sense of depth of the background image and the object image by maintaining a particular distance with respect to the focal length of the double Fresnel lens structure. Also, in order to adjust the size of the 3-dimensional image 9 and the sense of depth provided by the filter 8 in FIG. 1, a third Fresnel lens 3 may be used with the object image source supply part 5, between the reflector 6 and the half mirror 4, and its position may be controlled to effect the adjustment.
As described so far, the conventional technologies require two mutually independent image sources when creating a 3-dimensional image having a background display, and a particular distance must be maintained with respect to the object image source supply part 5. There is also a problem that when large devices are used, the distance from the position of the double Fresnel lenses to the object image source supply part 5 forms a very wide projection distance, to increase the overall volume of the device. This is because of an inherent property of Fresnel lenses that the image source supply part must maintain a certain distance with respect to the certain focal length of the double Fresnel lenses.